Metal halide lamps comprising a discharge vessel comprising a ceramic body are known in the art and are described in, for instance, US2009/0269523 and EP215524. Such lamps operate under high pressure and comprise ionizable gas fillings of, for instance, NaI (sodium iodide), TlI (thallium iodide), Cal2 (calcium iodide) and REI3. REI3 refers to rare-earth iodides. Characteristic rare-earth iodides for metal halide lamps are CeI3, PrI3, NdI3, DyI3 and LuI3 (cerium, praseodymium, neodymium, dysprosium and lutetium iodide, respectively). Typically said ceramic body is made of translucent gastight (poly) crystalline alumina (TGA), aluminumnitride, or yttrium-aluminum-garnet.
There is a continuous effort in industry to optimize such lamps and their production process. Lifetime and energy-saving aspects of the lamps as well as reduction of costs involved in the production process of the lamp are items that are investigated.
One specific item of interest is the lifetime of the lamp. Substantially long lifetimes are desired, without, however, a substantial change of lamp characteristics. Another item of interest is, for instance, the reduction of costs during the production process. For instance, lowering the heating temperature during a sealing stepin the production process, for example of sealing a feedthrough with its sealing part in an end part, might be of interest in view of saving costs. In the present production process of metal halide lamps, the lamps are sealed at relatively high temperatures. A reduction of heating time and/or heating temperature would be beneficial for the apparatus used for performing such a sealing step, but might also be beneficial for the lifetime of the lamp (less risk of crack formation).
A further specific item of interest to increase said lifetime is matching the thermal coefficient of expansion of the material of the seal with the material to seal, for example with the material of the sealing part and/or the ceramic material of the discharge vessel. In general, the better the match, the longer the lifetime and/or the less risk of defective lamps in modern lamp production processes of large quantities on an industrial scale. A better match will also reduce the risk of crack formation. Though Niobium is not very well resistant against the metal halide filling, Niobium is yet selected in the known lamp as sealing part because it has a coefficient of linear thermal expansion corresponding to that of the translucent sintered Al2O3 ceramic material of the discharge vessel, i.e. 7.3*10−6K−1 respectively 7.0*10−6K−1 (at 300K). Said sealing part is sealed and completely covered with a sealing compound into the respective end part of the lamp.
It is a disadvantage of the known lamp that both the sealing compound is not resistant against the metal halide filling, resulting in the disadvantage of the known lamp to be relatively long due to its long end parts, also called extended plug (or vup). The long end parts are desired as to keep the sealing compound at a relatively low temperature and thus to slow down the chemical attack process by the filling and to improve the stability of lamp characteristics and/or to extend the lifetime of the lamp.
In an alternative embodiment of the known lamp, as disclosed in WO2008075273, the sealing part is made of Iridium metal which is directly sealed to the ceramic material of the end part via shrink sealing. An improved resistance of the feedthrough construction against the corrosive metal halide filling is obtained as Iridium is resistant to said metal halide and the sealing compound is absent. However, this embodiment of the known lamp has the disadvantage that due to the shrink sealing process the Iridium has become (somewhat) brittle and less robust against shock, involving the disadvantage of an enhanced risk on short lifetime of the lamp. Moreover, Iridium metal is an expensive material rendering the lamp to be relatively expensive.